This invention generally relates to electrically powered machines, and more particularly to a method for controlling a reduced voltage motor starter for starting electrically-powered machines, such as electrically powered rotary or centrifugal compressors, for example.
Compressors are often driven by an electrically powered prime mover, such as a three-phase electric motor. These electric motors include an electric system which may utilize various electromechanical methods of reduced voltage motor starting.
Generally, reduced voltage motor starters (reduced voltage controllers) are employed to reduce the magnitude of line current drawn at the starting of a large horsepower electric motor, to reduce the starting torque of the motor applied to the driven machine, or both. Consequently, reduced voltage controllers may reduce the stress a large horsepower electric motor places on an electrical system. For example, the starting of a large horsepower electric motor can cause a large voltage drop in the facility's electrical system, thus reducing the available torque for accelerating the compressor load. In this regard, if the torque required to accelerate the compressor load is limited, then the motor will take longer to come up to a steady state speed, which may permit a transient torque condition to occur at starter transition which may damage the compressor. These torque transient conditions or reverse torque transients are caused by the tendency of the three-phase motor to shift phases which produces a very large spike in the current, thereby producing an opposite torque when a connection contactor is closed. Therefore, high impact loadings and great strains are placed on a compressor drive system which may exceed gear design strengths, and which may even cause compressor drive couplings and drive gears to shear.
Several methods of reduced voltage motor starting are available for use with large horsepower electric motors, and each method exhibits different torque-current characteristics. For example, some reduced voltage controllers lower the starting current and torque by means of an impedance inserted in the start-up circuitry of the electric motor. This impedance is mainly in the form of reactance. Many times, autotransformers are used to accomplish this type of reduced voltage starting. Another common method used to reduce electric motor start-up requirements is accomplished by a wye-delta (star-delta) type reduced voltage controller, or a partial winding reduced voltage controller. Depending on the type of controller used, the electric motor will produce a different amount of torque on starting for a given value of current drawn from the line.
Typically, each of these reduced voltage starting techniques requires an end user to adjust a mechanical timer or timing relay to provide a predetermined delay for transitioning the electromechanical contactors from a start configuration to a run configuration. This timing relay often requires exact adjustment to match the acceleration time of the electric motor and the machine, e.g. a rotary or centrifugal compressor, to perform a proper transition of the electromechanical contactors. However, during compressor operation, the timing relay often becomes improperly adjusted, which may cause damage to the compressor or the electrical system. Additionally, in the case of a wye-delta starter, the starter change over is arranged to occur in a fixed time regardless of actual motor speed. Accordingly, because torque transients can occur if the motor is not operating at a predetermined speed, fixed starter change over times may actually cause damage to the driven machine.
The foregoing illustrates limitations known to exist in present reduced voltage controllers. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.